The basic parameter you're interested in when doing macrophotography is the magnification. In fact, the word macro itself refers to big magnifications. And one of the cheapest of easy ways to achieve high magnification is reverse attaching a lens.

Best results are achieved with wide angle lens as the one reverse attached, but those tend to have small diameters. This can lead to some serious vignetting. And beware: even if you don't see vignetting in the eyepiece (when wide open) it doesn't mean that it won't be there on the photo (stepped down).

Example:

Pentax SMC M 50mm f/1.7 with MC Mitakon 28mm f/2.8 reverse attached, wide open:

Same setup, stepped down:

Note it's not centered appropriately - this has broken the sharpness, but vignetting will be the same even when the lens is mounted flawlessly.